System and Method for a Differential Comb Drive MEMS

ABSTRACT

According to an embodiment, a MEMS device includes a deflectable membrane including a first plurality of electrostatic comb fingers, a first anchor structure including a second plurality of electrostatic comb fingers interdigitated with a first subset of the first plurality of electrostatic comb fingers, and a second anchor structure including a third plurality of electrostatic comb fingers interdigitated with a second subset of the first plurality of electrostatic comb fingers. The second plurality of electrostatic comb fingers are offset from the first plurality of electrostatic comb fingers in a first direction and the third plurality of electrostatic comb fingers are offset from the first plurality of electrostatic comb fingers in a second direction, where the first direction is different from the second direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/928,702, filed on Oct. 30, 2015, which application is herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to transducers, and, inparticular embodiments, to a system and method for a differential combdrive MEMS.

BACKGROUND

Transducers convert signals from one domain to another and are oftenused in sensors. One common transducer operating as a sensor that isseen in everyday life is a microphone, which converts, i.e., transduces,sound waves into electrical signals. Another example of a common sensoris a thermometer. Various transducers exist that serve as thermometersby transducing temperature signals into electrical signals.

Microelectromechanical system (MEMS) based transducers include a familyof sensors and actuators produced using micromachining techniques. MEMSsensors, such as a MEMS microphone, gather information from theenvironment by measuring the change of physical state in the transducerand transferring a transduced signal to processing electronics that areconnected to the MEMS sensor. MEMS devices may be manufactured usingmicromachining fabrication techniques similar to those used forintegrated circuits.

MEMS devices may be designed to function as, for example, oscillators,resonators, accelerometers, gyroscopes, pressure sensors, microphones,and micro-mirrors. Many MEMS devices use capacitive sensing techniquesfor transducing the physical phenomenon into electrical signals. In suchapplications, the capacitance change in the sensor is converted to avoltage signal using interface circuits.

One such capacitive sensing device is a MEMS microphone. A MEMSmicrophone generally has a deflectable membrane separated by a smalldistance from a rigid backplate. In response to a sound pressure waveincident on the membrane, it deflects towards or away from thebackplate, thereby changing the separation distance between the membraneand backplate. Generally, the membrane and backplate are made out ofconductive materials and form “plates” of a capacitor. Thus, as thedistance separating the membrane and backplate changes in response tothe incident sound wave, the capacitance changes between the “plate” andan electrical signal is generated.

MEMS microphones with this type of parallel plate capacitive structureformed from the deflectable membrane and rigid backplate may includevarious performance characteristics as a consequence of the parallelplate structure. For example, the rigid backplate is often perforated inorder to allow air to pass through the backplate so that the rigidbackplate is acoustically transparent. However, in practice, the rigidbackplate often is not fully acoustically transparent and generates someamount of acoustic noise. This often leads to a tradeoff betweenmechanical robustness, such as by including fewer and smallerperforations in the rigid backplate, and acoustic noise reduction, suchas by including more and larger perforations in the rigid backplate.

Another characteristic of such parallel plate structures is thephenomenon known as “pull-in.” In order to operate as an acoustictransducer, a bias voltage is applied between the deflectable membraneand the rigid backplate. Because of the voltage applied between theplates, changes in capacitance between the plates, resulting from motionof the deflectable membrane, produce a measurable voltage signal thatcorresponds to an incident acoustic signal. However, due to the appliedbias voltage, as the separation distance between the deflectablemembrane and the rigid backplate decreases, an attractive electrostaticforce also increases. The attractive electrostatic force is usuallybalanced by a restoring mechanical spring force in the deflectablemembrane, the attractive electrostatic force increases non-linearly asthe distance becomes small while the restoring mechanical spring forceincreases only linearly. The difference in relation to separationdistance results in the attractive electrostatic force overcoming therestoring mechanical spring force when the separation distance reaches acertain limit, which causes pull-in or collapse as the deflectablemembrane moves all the way to contact the rigid backplate and may resultin stiction. The phenomenon of pull-in presents another tradeoff betweenresistance to pull-in, from increased rigidity of the deflectablemembrane or lower bias voltage, and higher sensitivity, from reducedrigidity of the deflectable membrane or increased bias voltage.

As a further example, dual backplate MEMS microphones are used in orderto generate differential signals. Dual backplate MEMS microphonesinclude a deflectable membrane, similar to standard parallel platemicrophone, and also include both a top backplate and a bottom backplateabove and below, respectively, the deflectable membrane. Thus, as thedeflectable membrane moves, the capacitance between the deflectablemembrane and one of the two backplates increases while the capacitancebetween the deflectable membrane and the other of the two backplatesdecreases. Such structures also exhibit the noise characteristicsresulting from perforations in the rigid backplates and are susceptibleto the phenomenon of pull-in as described hereinabove.

SUMMARY

According to an embodiment, a MEMS device includes a deflectablemembrane including a first plurality of electrostatic comb fingers, afirst anchor structure including a second plurality of electrostaticcomb fingers interdigitated with a first subset of the first pluralityof electrostatic comb fingers, and a second anchor structure including athird plurality of electrostatic comb fingers interdigitated with asecond subset of the first plurality of electrostatic comb fingers. Thesecond plurality of electrostatic comb fingers are offset from the firstplurality of electrostatic comb fingers in a first direction and thethird plurality of electrostatic comb fingers are offset from the firstplurality of electrostatic comb fingers in a second direction, where thefirst direction is different from the second direction.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment transducer system;

FIGS. 2 a, 2 b, and 2 c illustrate a top view, a first cross sectionalview, and a second cross sectional view of an embodiment transducer;

FIGS. 3a and 3b illustrate top views of further embodiment transducers;

FIG. 4 illustrates a top view of another embodiment transducer;

FIG. 5 illustrates a flow chart diagram of an embodiment method offabrication for an embodiment transducer;

FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, and 6 f illustrate cross sectional viewsof portions of embodiment transducers;

FIG. 7 illustrates a cross sectional view of still another embodimenttransducer; and

FIG. 8 illustrates a flow chart diagram of another embodiment method offabrication for an embodiment transducer.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Description is made with respect to various embodiments in a specificcontext, namely microphone transducers, and more particularly, MEMSmicrophones. Some of the various embodiments described herein includeMEMS transducer systems, MEMS microphone systems, MEMS microphonesproducing differential signals, and interdigitated comb drive MEMStransducers. In other embodiments, aspects may also be applied to otherapplications involving any type of transducer according to any fashionas known in the art.

As described hereinabove in the background, various MEMS transducerswith parallel plate capacitive structures exhibit certaincharacteristics and tradeoffs due to the parallel plate structure.According to various embodiments described herein, a MEMS transducerincludes a deflectable membrane with a comb drive structure for sensingor actuation. In such embodiments, the MEMS transducer may include thedeflectable membrane without any sensing backplate. The MEMS transducerhas a comb drive, or comb drives, along one or more edges of thedeflectable membrane.

In various embodiments, each comb drive portion includes a first stator,a second stator, and a rotor, each with a plurality of comb fingers. Insuch embodiments, the first stator is connected to an anchor and offsetout-of-plane from the deflectable membrane in a first direction and thesecond stator is connected to an anchor and offset out-of-plane from thedeflectable membrane in a second direction that is opposite the firstdirection. The rotor is connected to an edge of the membrane.

In such various embodiments, the offsets in the first and seconddirection, such as downward and upward, for example, may be caused byforming extension or supporting portions of the first stator and thesecond stator with an internal or intrinsic layer stress. For example,the extension of supporting portions between the comb fingers and theanchor structures may include different material layers with variouspatterns to produce at rest and unbiased deflection of the first andsecond stators in different directions, such as upwards and downwards.In such embodiments, by having offsets in different directions of twostators electrostatically coupled to the rotor, embodiment MEMStransducers generate a differential transduced signal based on motion ofthe comb fingers in the rotor that is part of the deflectable membrane.Further details of specific embodiments are described hereinafter inreference to the figures.

FIG. 1 illustrates a block diagram of an embodiment transducer system100 including differential MEMS acoustic transducer 102, applicationspecific integrated circuit (ASIC) 104, and acoustic processor 106.According to various embodiments, differential MEMS acoustic transducer102 is coupled to an external environment through sound port 108.Acoustic signals transfer through sound port 108 to differential MEMSacoustic transducer 102. In such embodiments, differential MEMS acoustictransducer 102 is fluidically coupled to the external or ambientenvironment through sound port 108. Acoustic signals are thus transducedat differential MEMS acoustic transducer 102. In various embodiments,acoustic transduction may include generation of acoustic signals fromelectrical signals, such as through actuation in a MEMS microspeaker, orgeneration of electrical signal from acoustic signals, such as throughsensing in a MEMS microphone.

According to various embodiments, sound waves incident on differentialMEMS acoustic transducer 102 through sound port 108 are transduced intoa differential signal. In specific embodiments, the differential signalis an analog signal including first analog signal SA+ and second analogsignal SA−, which are provided to ASIC 104 for amplification and analogto digital conversion. In such embodiments, ASIC 104 includes anamplification stage and an analog-to-digital converter (ADC). Based onfirst analog signal SA+ and second analog signal SA−, ASIC 104 generatesdigital signal SD and provides digital signal SD to acoustic processor106. In various other embodiments, ASIC 104 includes two amplificationstages for generating a differential signal that is provided to acousticprocessor 106. In still other embodiments, ASIC 104 includes twoamplification stages and two analog-to-digital converters (ADCs) forgenerating the differential digital signal that is provided to acousticprocessor 106. In still further embodiments, ASIC 104 includes twoamplification stages and an analog 180° combiner that together generatea single-ended signal that is provided to acoustic processor 106.

In various other embodiments, transducer system 100 may operate as amicrospeaker, i.e., in reverse. In such embodiments, acoustic processor106 generates digital signal SD and provides digital signal SD to ASIC104, which in turn generates first analog signal SA+ and second analogsignal SA− as drive signals for differential MEMS acoustic transducer102. Based on first analog signal SA+ and second analog signal SA− asdrive signals, differential MEMS acoustic transducer 102 generatesacoustic signals, e.g., sound waves, which propagates through sound port108 to the external or ambient environment.

In various embodiments, differential MEMS acoustic transducer 102includes a MEMS device structure that has a first stator and a secondstator electrostatically coupled to a rotor attached to a deflectablemembrane. The first stator and the second stator have differentpositions relative to the rotor in order to generate a differentialsignal based on motions of the deflectable membrane.

Transducer system 100 may include numerous modifications in differentembodiments. For example, certain embodiments include a singleintegrated circuit (IC) that include differential MEMS acoustictransducer 102 and ASIC 104 integrated on a single die, such as asemiconductor die. In alternative embodiments, differential MEMSacoustic transducer 102 is a digital MEMS microphone that generates adigital signal instead of an analog signal. In some embodiments,acoustic processor 106 is omitted. In specific embodiments, acousticprocessor 106 is a custom audio processor. For example, acousticprocessor 106 may be a coder/decoder (codec). In various embodiments,ASIC 104 may a fully custom IC, a partially custom IC, or anoff-the-shelf IC. In some embodiments, ASIC 104 may provide a biasvoltage or bias voltages to differential MEMS acoustic transducer 102.In some further embodiments, ASIC 104 may perform additional functions(not shown), such as calibration, self-test, diagnostic, or repairfunctions, for example.

Transducer system 100 may be packaged in a plastic, glass, or metalpackage in some embodiments. Differential MEMS acoustic transducer 102may be separately packaged in a plastic, glass, or metal microphonepackage that includes sound port 108. In such embodiments, ASIC 104 maybe included in the microphone package or packaged separately. Forexample, differential MEMS acoustic transducer 102, ASIC 104, andacoustic processor 106 may each be packaged separately and attached to aprinted circuit board (PCB) that provides electrical coupling betweenthe components of transducer system 100. In various embodiments,transducer system 100 may be included in a personal mobile device, suchas tablet, smart phone, laptop, smartwatch, or other wearable device. Inother embodiments, transducer system 100 may be included in largermobile systems, such as an automotive or other vehicle system, forexample. In still other embodiments, transducer system 100 may beincluded in a non-mobile system, such as a smart home or residentialenvironment or in an industrial setting. In yet further embodiments,transducer system 100 may be included in robotic systems. In otherembodiments, transducer system 100 may be included in any type ofsystem.

FIGS. 2 a, 2 b, and 2 c illustrate a top view, a first cross sectionalview, and a second cross sectional view of an embodiment transducer 110a including substrate 112, membrane 114, negative stator 116, andpositive stator 118. FIG. 2a illustrates the top view of transducer 110a, FIG. 2b illustrates the cross sectional view at cross-section 111 a,and FIG. 2c illustrates the second cross sectional view at cross-section111 b. According to various embodiments, membrane 114 is a deflectablerectangular flap membrane anchored to substrate 112 along one edge andelectrostatically coupled along an opposite edge to negative stator 116and positive stator 118 through interdigitated comb fingers 120 a andinterdigitated comb fingers 120 b, which operate as an electrostaticcomb drive. In such embodiments, positive stator 118 is deflectedout-of-plane (referring to a plane including the top surface of membrane114) in an upward direction as shown at cross-section 111 a asillustrated by FIG. 2 b, and negative stator 116 is deflectedout-of-plane (referring to a plane including the top surface of membrane114) in a downward direction as shown at cross-section 111 b asillustrated by FIG. 2 c.

In various embodiments, pressure waves, such as fluidic signalsincluding sound waves, incident on membrane 114 produce deflections ofmembrane 114. As membrane 114 deflects due to the incident sound waves,for example, the deflections cause the parallel plate capacitancebetween interdigitated comb fingers 120 a and interdigitated combfingers 120 b to change. For example, when an incident sound wave causesmembrane 114 to move down, the parallel plate capacitance betweeninterdigitated comb fingers 120 a and interdigitated comb fingers 120 bincreases for negative stator 116 and decreases for positive stator 118.In such embodiments, the plate overlap (as viewed through the crosssectional view of FIG. 2b ) between interdigitated comb fingers 120 aand interdigitated comb fingers 120 b at positive stator 118 decreasesas membrane 114 moves downward, leading to the decrease in parallelplate capacitance for positive stator 118. Similarly, the plate overlap(as viewed through the cross sectional view of FIG. 2c ) betweeninterdigitated comb fingers 120 a and interdigitated comb fingers 120 bat negative stator 116 increases as membrane 114 moves downward, leadingto the increase in parallel plate capacitance for negative stator 116.

Further, as membrane 114 moves upward, the plate overlap (as viewedthrough the cross sectional view of FIG. 2b ) between interdigitatedcomb fingers 120 a and interdigitated comb fingers 120 b at positivestator 118 increases, leading to an increase in parallel platecapacitance for positive stator 118. Similarly, as membrane 114 movesupward, the plate overlap (as viewed through the cross sectional view ofFIG. 2c ) between interdigitated comb fingers 120 a and interdigitatedcomb fingers 120 b at negative stator 116 decreases, leading to adecrease in parallel plate capacitance for negative stator 116.

According to various embodiments, because the parallel plate capacitancefor negative stator 116 and positive stator 118 exhibit inverse changes,a differential signal is generated by negative stator 116 and positivestator 118 as membrane 114 deflects. In such embodiments, thedifferential signal corresponds to the pressure signal, such as acousticsignals for example. The differential signal may be read out as adifferential voltage signal from negative stator 116 and positive stator118 in some embodiments. In some embodiments, membrane 114 may deflectfurther until the plate overlap between interdigitated comb fingers 120a and interdigitated comb fingers 120 b (as viewed through the crosssectional views of FIGS. 2b and 2c ) begins to decrease for bothnegative stator 116 and positive stator 118.

According to alternative embodiments, a voltage signal is applied tointerdigitated comb fingers 120 a and interdigitated comb fingers 120 bin order to generate an electrostatic force on membrane 114 and causedeflections. In such alternative embodiments, membrane 114 may beexcited to generate pressure waves, such as sound waves. Theelectrostatic force is generated by the interaction of interdigitatedcomb fingers 120 a and interdigitated comb fingers 120 b operating as anelectrostatic comb drive. In such alternative embodiments, transducer110 a may operate as a microspeaker.

In various embodiments, positive stator 118 is deflected at rest upwardand negative stator 116 is deflected at rest downward. In suchembodiments, interdigitated comb fingers 120 a of positive stator 118are supported by, and electrically coupled to conductive layer 132,which is fixed in substrate 112 on one end. Similarly, interdigitatedcomb fingers 120 a of negative stator 116 are supported by, andelectrically coupled to conductive layer 132, which is fixed insubstrate 112 on one end. Conductive layer 132 is sandwiched betweenbottom stress layer 130 and top stress layer 134. In variousembodiments, bottom stress layer 130 and top stress layer 134 eachinclude tensile or compressive layer stress that pulls or pushesconductive layer 132 towards one of the stress layers. In regions wherebottom stress layer 130 and top stress layer 134 are included below andabove, i.e., sandwiching conductive layer 132, the tensile orcompressive stress that pulls or pushes conductive layer 132 pulls orpushes equally in both directions. In such embodiments, the deflectingforce is balanced and conductive layer 132 does not deflect at rest. Inareas where either bottom stress layer 130 or top stress layer 134 isremoved, while the other stress layer is included, the force is notbalanced and conductive layer 132 deflects or curves towards or awayfrom the respective stress layer that is included, depending on whetherthe stress is compressive or tensile. The discussion hereinafter refersto bottom stress layer 130 and top stress layer 134 as compressivestress layers.

Specifically, in some embodiments, bottom stress layer 130 for positivestator 118 includes patterned opening 124. In the region above patternedopening 124, conductive layer 132 curves upward toward top stress layer134. Similarly, top stress layer 134 includes patterned opening 122. Inthe region below patterned opening 122, conductive layer 132 curvesdownward toward bottom stress layer 130. In such embodiments, positivestator 118 is deflected upward at rest and without a voltage bias due tothe patterning of bottom stress layer 130 or top stress layer 134.

In further specific embodiments, bottom stress layer 130 for negativestator 116 includes patterned opening 128. In the region above patternedopening 128, conductive layer 132 curves upward toward top stress layer134. Similarly, top stress layer 134 includes patterned opening 126. Inthe region below patterned opening 126, conductive layer 132 curvesdownward toward bottom stress layer 130. In such embodiments, negativestator 116 is deflected downward at rest and without a voltage bias dueto the patterning of bottom stress layer 130 or top stress layer 134.

In other embodiments, bottom stress layer 130 and top stress layer 134may include tensile stress. In such embodiments, conductive layer 132curves in the opposite direction in patterned openings. For example,when bottom stress layer 130 has tensile stress, conductive layer 132curves upward at patterned opening 122 and downward at patterned opening124 (opposite as shown). In various embodiments, the type of layerstress may be tensile or compressive, and is dependent on the materialused and the method of depositing or forming the material, as willreadily be appreciated by those having skill in the art.

In various embodiments, insulating layer 136 and insulating layer 138are formed in contact with substrate 112. In some embodiments,insulating layer 136 and insulating layer 138 are oxide layers, nitridelayers, or oxynitride layers. In a specific embodiment, insulating layer136 is silicon oxide and insulating layer 138 is silicon nitride. Inother embodiments, insulating layer 136 and insulating layer 138 may beother types of dielectric materials. In various embodiments, substrate112 is a support layer formed on another substrate (not shown; see FIG.7 for example). Substrate 112 includes cavity 113 below membrane 114. Inembodiments where substrate 112 is a supporting layer formed on anadditional substrate, cavity 113 may extend into the additionalsubstrate that substrate 112 is formed on top of (not shown; seesubstrate 174 in FIG. 7 for example). In such embodiments, substrate 112may be a tetraethyl orthosilicate (TEOS) oxide layer or anotherinsulating structural layer. In some embodiments, substrate 112 orsubstrate 174 is a semiconductor substrate, such as silicon, silicongermanium, or carbon, for example. In still further embodiments,substrate 112 may be a glass substrate or a plastic substrate.

According to various embodiments, conductive layer 132 may includesemiconductor materials or metals. In a specific embodiment, conductivelayer 132 is polysilicon. In another embodiment, conductive layer 132 issingle-crystal silicon. In an alternative embodiment, conductive layer132 is aluminum. Similarly, membrane 114 may be formed of the samematerial and at the same time as conductive layer 132. For example,membrane 114 is formed during a deposition step for forming conductivelayer 132.

In various embodiments, bottom stress layer 130 and top stress layer 134each include an insulating material. In some embodiments, bottom stresslayer 130 and top stress layer 134 include an insulating material with adifferent intrinsic layer stress than conductive layer 132. In aspecific embodiment, bottom stress layer 130 and top stress layer 134are silicon nitride. In other embodiments, bottom stress layer 130 andtop stress layer 134 are another type of dielectric material. Furtherdiscussion of embodiment materials and methods of fabricating embodimenttransducers is provided hereinafter in reference to FIG. 5, for example.

In various embodiments, positive stator 118 is electrically isolatedfrom negative stator 116. Metallization and vias (not shown) may provideelectrical connections between separate contacts (not shown) to membrane114, conductive layer 132 of positive stator 118, and conductive layer132 of negative stator 116.

In various embodiments, transducer 110 a may operate with any type offluidic medium. In a particular embodiment, transducer 110 a, andspecifically membrane 114, interacts with air and pressure waves, e.g.,sound waves, propagating in the air. In other embodiments, other mediawith pressure waves may interact with transducer 110 a.

As shown in FIGS. 2b and 2 c, cross-sections 111 a and 111 b include across-section that is not a straight line for illustration purposes.Specifically, cross-sections 111 a and 111 b each show one ofinterdigitated comb fingers 120 b. Due to the different position of theinterdigitated comb fingers, a straight line cross-section would notpass through both interdigitated comb fingers 120 a and interdigitatedcomb fingers 120 b. Thus, this illustration is presented in order toimprove understanding. Further, substrate 112 is illustrated as the toplayer in FIG. 2 a, but substrate 112 may include insulating layer 136and insulating layer 138 on top of it. Substrate 112 is illustrated inFIG. 2a in order to depict the structural layer that supports membrane114, positive stator 118, and negative stator 116. In variousembodiments, substrate 112 may include metallization, contact pads, andother insulating layers on a top surface of substrate 112.

FIG. 3a illustrates a top view of another embodiment transducer 110 bincluding substrate 112, membrane 114, negative stator 116, and positivestator 118. According to various embodiments, transducer 110 b issimilar to transducer 110 a described hereinabove in reference to FIGS.2 a, 2 b, and 2 c, with a different type of membrane 114. In suchembodiments, membrane 114 is a square, or rectangular, membrane anchoredat each of the four corners by support beams 140. Membrane 114 includesinterdigitated comb fingers 120 b on each of the four sides. Transducer11 b also includes four instances of negative stator 116, one on each ofthe four sides, and four instances of positive stator 118, one on eachof the four sides, each of the stator structures includinginterdigitated comb fingers 120 a interdigitated with interdigitatedcomb fingers 120 b of membrane 114.

In various embodiments, description of the elements of transducer 110 aalso applies to similar numbered elements of transducer 110 b and willnot be repeated in the interest of brevity. Support beams 140 may beformed of the same layer as membrane 114. In other embodiments, supportbeams 140 may be thicker than the central portion of membrane 114.According to various embodiments, using interdigitated comb fingers 120a and interdigitated comb fingers 120 b on all four sides of membrane114 may increase the sensitivity of transducer 110 b.

According to various embodiments, embodiment transducers describedherein, such as transducer 110 b, for example, may have any shape withany configuration of anchors in different embodiments. Specifically,transducer 110 b includes a square membrane. According to otherembodiments, transducers may have any shape. In particular embodiments,transducers may have a round shape, an oval shape, or a polygon shape.FIG. 3b illustrates a top view of another embodiment transducer 110 c assimilarly described hereinabove in reference to transducer 110 a andtransducer 110 b in FIGS. 2 a, 2 b, 2 c, and 3 a with a specific shape.In such embodiments, membrane 114 is an octagon anchored at each of theeight vertexes by support beams 140. In other embodiments, transducer110 c (and correspondingly cavity 113 and membrane 114) has a shape thatmay be any type polygon, such as a triangle, square, pentagon, hexagon,heptagon, octagon, and so on. In further embodiments, transducer 110 cmay have a round shape. Those having skill in the art will readilyappreciate that various different shapes may include variousconfigurations of support beams 140 and instances of negative stator 116and positive stator 118.

FIG. 4 illustrates a top view of a further embodiment transducer 110 dincluding substrate 112, membrane 114, negative stator 116, and positivestator 118. According to various embodiments, transducer 110 d issimilar to transducer 110 a described hereinabove in reference to FIGS.2 a, 2 b, and 2 c, with a different type of membrane 114. In suchembodiments, membrane 114 is a square, or rectangular, membrane anchoredalong two opposite sides. Membrane 114 includes interdigitated combfingers 120 b on the other sides. Transducer 110 d also includes sixinstances of negative stator 116, three on each of two opposite sides,and six instances of positive stator 118, three on each of the twoopposite sides, each of the stator structures including interdigitatedcomb fingers 120 a interdigitated with interdigitated comb fingers 1120b of membrane 114. In other embodiments, any number of instances ofpositive stator 118 and negative stator 116 may be included. In variousembodiments, positive stator 118 and negative stator 116 may eachinclude 2, 4, 6, 8, 10, 12, 14, 16, 18, or 20 instances, half on each ofthe two opposite sides. In other specific embodiments, more than 20instances of positive stator 118 and negative stator 116 may beincluded. In other specific embodiments, one instance of positive stator118 covers one side and one instance of negative stator 116 covers theopposite site.

In various embodiments, description of the elements of transducer 110 aalso applies to similar numbered elements of transducer 110 d and willnot be repeated in the interest of brevity. According to variousembodiments, using interdigitated comb fingers 120 a and interdigitatedcomb fingers 120 b on two sides of membrane 114 may increase thesensitivity of transducer 110 d. Transducer 110 d is shown withinterdigitated comb fingers 120 a, and correspondingly positive stators118 and negative stators 116, arranged close to the edge to whichmembrane 114 is anchored. In other embodiments, interdigitated combfingers 120 a, and correspondingly positive stators 118 and negativestators 116, may be arranged closer to the central region and not nearthe edges to which membrane 114 is anchored.

Transducer 110 a, transducer 110 b, transducer 110 c, and transducer 110d as illustrated in FIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4 each includepositive stator 118, negative stator 116, and membrane 114 with aspecific number of interdigitated comb fingers 120 a and interdigitatedcomb fingers 120 b. In other embodiments, any number of interdigitatedcomb fingers 120 a and interdigitated comb fingers 120 b may beincluded. In specific embodiments, the number of interdigitated combfingers 120 a and the number of interdigitated comb fingers 120 b rangefrom 3 to 100. In general, the number of interdigitated comb fingers 120a is two larger than the number of interdigitated comb fingers 120 b foreach edge, or vice versa.

Further, in various embodiments, interdigitated comb fingers 120 a andinterdigitated comb fingers 120 b each have a length ranging from 1 μmto 50 μm. Further, the layer thickness of interdigitated comb fingers120 a and interdigitated comb fingers 120 b may range from 1 μm to 50μm. In various embodiments, the width of interdigitated comb fingers 120a and interdigitated comb fingers 120 b ranges from 1 μm to 10 μm.

FIG. 5 illustrates a flow chart diagram of an embodiment method offabrication 200 for an embodiment transducer. Method of fabrication 200includes steps 205, 210, 215, 220, 225, 230, 235, 240, 245, and 250.According to various embodiments, method of fabrication 200 is a methodof forming any of transducer 110 a, transducer 110 b, transducer 110 c,and transducer 110 d, for example. In various embodiments, step 205includes forming trenches in a first main surface of a substrate. Thesubstrate may include a semiconductive material such as silicon orgermanium, or a compound semiconductor such as SiGe, GaAs, InP, GaN orSiC. In alternative embodiments, the substrate may include organicmaterials such as glass or ceramic. The substrate may be a wafer.

The trenches may be etched in a first main surface of the substrate. Thetrenches may be etched by applying a wet etch chemistry or a dry etchchemistry. For example, the trenches may be etched by applying areactive ion etch (RIE) process. The trenches may be staggered in orderto form both sets of interdigitated comb fingers in a comb drive, suchas interdigitated comb fingers 120 a and interdigitated comb fingers 120b as described hereinabove in reference to FIGS. 2 a, 2 b, 2 c, 3 a, 3b, and 4. The stator is offset from the membrane. Between the trenchesetched in the substrate, rims or fins of un-etched material separate thetrenches.

In various embodiments, the bottom surface and the sidewalls of thetrenches formed in step 205 and the top surface of the substrate arecovered with an insulating layer in step 210. Specifically, step 210includes depositing the insulating layer in the trenches. The insulatinglayer may include an oxide layer, a nitride layer and/or an oxynitridelayer. For example, the insulating layer may be a silicon oxide or aTEOS oxide layer. Alternatively, the insulating layer may be a siliconnitride layer. The insulating layer may be deposited or grown as aconformal layer in step 210. The insulating layer may be deposited suchthat the insulating layer covers only the bottom surface and thesidewalls of the trenches but not a central portion of the trenches. Insome embodiments, the trenches are partially filled with the insulatinglayer. In some embodiments, the insulating material of the insulatinglayer may be deposited by applying a chemical vapor deposition (CVD)process, a physical vapor deposition (PVD) process, an atomic layerdeposition (ALD) process, or a wet or dry oxidation of the substrate.

According to various embodiments, step 215 includes depositing a firststress layer. The first stress layer may be deposited using any of theprocessing techniques described in reference to step 210. The firststress layer may include materials deposited with tensile stress orcompressive stress. In various embodiments, the first stress layer is ahigh stress material. In a specific embodiment, the first stress layeris silicon nitride (SiN) having a tensile stress of about 1 GPa. Inanother specific embodiment, the first stress layer is siliconoxynitride (SiON) having a tensile stress ranging from about 400 MPa toabout 800 MPa. In other embodiments, the first stress layer is a lowstress material. In a specific embodiment, the first stress layer isTEOS having a compressive stress of about 100 MPa. In another specificembodiment, the first stress layer is silicon (Si) having a compressivestress ranging from about 100 MPa to about 50 MPa, which may depend onthe dopant, such as phosphorous (P), implantation, for example. Invarious embodiments, the first stress layer and the insulating layer maybe the same layer.

In various embodiments, step 220 includes patterning the first stresslayer formed in step 215. Patterning the first stress layer may includeapplying a photoresist, developing the photoresist using a mask pattern,and etching the first stress layer in the exposed regions. Etching thefirst stress layer may include a wet chemistry etch or a dry chemistryetch. The first stress layer may be etched in the trenches andeverywhere on the surface of the substrate except between the statorfingers and the substrate. For example, step 220 may include formingpatterned opening 124 and patterned opening 128 as described hereinabovein reference to FIGS. 2b and 2 c.

Step 225 includes depositing a conductive material in the trenches. Theconductive material may be a finger material for the interdigitated combfingers. In some embodiments, the conductive material may fill thetrenches. The conductive material may be a metallic material. Themetallic material may comprise a pure metal, an alloy and/or a compound.In some embodiments, the metallic material may, for example, include oneor more of the elements chosen from the group consisting of Al, Cu, Niand Si. Specific embodiments, include pure aluminum, aluminum alloy,aluminum compound, pure copper, copper alloy, copper compound, purenickel, nickel alloy and nickel compound. In one specific embodiment,the conductive material is AlSiCu. In other embodiments, the conductivematerial may include a conductive polymer. In still other embodiments,the conductive material includes a doped semiconductor such as dopedsilicon. The doped silicon may comprise doped polysilicon and/or dopedmonocrystalline silicon. The doped silicon may be in situ doped.

In various embodiments, the conductive material may be deposited indifferent ways such as sputtering, PVD, CVD or ALD. The conductivematerial may be deposited as a single step (for example, the trenchesmay be filled (e.g. completely filled) or in two or more steps. When theconductive material comprises a metallic material, it is possible thatthe conductive material is deposited by a galvanic deposition. Theconductive material may be directly deposited onto the insulating layerand the first stress layer. In addition to being deposited in thetrenches, the conductive layer may also be deposited to form themembrane, such as membrane 114 as described hereinabove in reference toFIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4.

In various embodiments, step 230 includes patterning the conductivematerial. Patterning the conductive material in step 230 may includeforming the interdigitated comb fingers, the membrane, the first statorand second stator (such as positive stator 118 and negative stator 116,respectively, as described hereinabove in reference to FIGS. 2 a, 2 b, 2c, 3 a, 3 b, and 4, for example), and the contacts, such as contact padsfor example. In such embodiments, step 230 includes depositing aphotoresist over the conductive material and structuring thephotoresist, such as by developing the photoresist using a mask pattern.The exposed portions of the conductive material are then removed. Theconductive material may be etched down to the insulating layer or thefirst stress layer in certain areas based on the patterned photoresist.In such embodiments, the conductive material formed in the trenches isnot removed. The conductive material in the trenches may form fingers,such as interdigitated comb fingers 120 a and interdigitated combfingers 120 b. In various embodiments, the conductive material may beremoved by applying a wet etch or a dry etch chemistry. For example,when the conductive material includes a semiconductor, e.g., a dopedsemiconductor such as doped silicon, the conductive material may beetched with KOH or acid solutions of HNO₃ and HF. In another embodimenta plasma process with chlorine or fluorine delivered by SF₆ or Cl₂ maybe used to remove the conductive material.

In various embodiments, the etch process of the conductive material maybe stopped when the top surface of the insulating layer or the firststress layer is reached. In some embodiments, the etch process isstopped either by end point detection or by timing (the layer thicknessof the insulating layer is much less than the depth of the fingers inthe trenches. In various embodiments, only the trenches are filled withthe conductive material and the first stress layer is deposited afterthe first conductive material and an additional conductive material isdeposited to form the membrane and the stator area or areas.

Step 230 may also include forming contact pads and the membrane. Themembrane and the contact pads may be formed in or on the substrate. Thecontact pads and the membrane include the conductive material. Inalternative embodiments, the contact pads may be silicided at thecontact pad locations. The silicided pads may formed by forming ametallic material on the conductive material. The metallic material mayinclude one or more of the elements from the group consisting of Ni, Co,and Ti. The conductive material and the metallic material may beannealed to form the silicide. In some embodiments the contact pads arepassivated.

Following forming the interdigitated comb fingers and the membrane instep 230, step 235 includes depositing the second stress layer.Depositing the second stress layer in step 235 may include all thefeatures described hereinabove in reference to depositing the firststress layer in step 215. Following depositing the second stress layerin step 235, step 240 includes patterning the second stress layer.AjPatterning the second stress layer in step 240 may be performed asdescribed hereinabove in reference to step 220. In various embodiments,the second stress layer may be patterned to be removed in the trenchesand everywhere on the surface of the substrate except between the statorfingers and the substrate. For example, step 240 may include formingpatterned opening 122 and patterned opening 126 as described hereinabovein reference to FIGS. 2b and 2 c.

According to various embodiments, step 245 includes etching thesubstrate from the back surface or backside. In such embodiments, thesubstrate is etched with a directional etch. For example, the substrateis etched with a Bosch process etch. This backside etch is applied suchthat the substrate is removed under the membrane formed and patterned insteps 225 and 230 (such as membrane 114 as described hereinabove inreference to FIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4) and such that thesubstrate remains under the stators. In specific embodiments, thebackside etch is stopped by the insulating layer of step 210. In suchembodiments, the interdigitated comb fingers are encased in theinsulating layer and remain standing and un-etched. In variousembodiments, step 245 includes forming the cavity beneath the membraneand the interdigitated comb fingers, such as described hereinabove inreference to cavity 113 in FIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4.

In alternative embodiments, the substrate backside is etched with a wetetch including, for example, KOH. In another embodiment the substratebackside is etched with a combination of dry etch to the level of thetrenches and subsequent wet etching with a higher selectivity of thesubstrate, such as a higher silicon selectivity, for example, versus theetch rate of the insulating layer.

According to various embodiments, step 250 includes removing theinsulating layer formed in step 210 using a release etch. In suchembodiments, the insulating layer is removed with a wet etch or a dryetch. For example, the insulating layer is etched by applying an HFbased solution or vapor. Following step 250, the transducer is releasedand the membrane, e.g., membrane 114, with the interdigitated combfingers, e.g., interdigitated comb fingers 120 b, is free to move.Further, following the release etch, the stators, such as positivestator 118 and negative stator 116 as described hereinabove in referenceto FIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4, may deflect to the at restposition. In specific embodiments, following the release etch,interdigitated comb fingers 120 a may deflect to an offset position withrespect to interdigitated comb fingers 120 b, as described hereinabove.In such embodiments, one, or some, of the stators, e.g., positive stator118, may deflect upwards and one, or some, of the stators, e.g.,negative stator 116, may deflect downwards. In such various embodiments,the stator fingers may be interlocked or interdigitated with themembrane fingers in the membrane. The cavity formed in step 245 islocated underneath the membrane so that the membrane can move up anddown relative to the stators.

Further modifications to the method of fabrication, including theaddition or substitution of process steps will be readily appreciated bythose having skill in the art. Additional description of processingsteps in reference to interdigitated comb finger transducers isdescribed in co-pending U.S. patent application Ser. No. 13/743,306,filed on Jan. 15, 2013 and entitled “Comb MEMS Device and Method ofMaking a Comb MEMS Device,” which is incorporated herein by reference inits entirety.

FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, and 6 f illustrate cross sectional viewsof portions of embodiment transducers. According to various embodiments,FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, and 6 f depict extension portions 150 a,150 b, 150 c, 150 d, 150 e, and 150 f extending from a substrate tointerdigitated comb fingers for a stator. For example, extensionportions 150 a, 150 b, 150 c, 150 d, 150 e, and 150 f may be consideredembodiment implementations of portions extending from substrate 112 tointerdigitated comb fingers 120 a of positive stator 118 or negativestator 116, as described hereinabove in reference to FIGS. 2 a, 2 b, 2c, 3 a, 3 b, and 4. Further, FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, and 6 f areillustrated in order to improve understanding in reference to method offabrication 200 described hereinabove in reference to FIG. 5.

In various embodiments, extension portions 150 a, 150 b, 150 c, 150 d,150 e, and 150 f include bottom structural layer 152, bottom stresslayer 154, conductive layer 156, top stress layer 158, and topstructural layer 160. In some embodiments, bottom structural layer 152and top structural layer 160 are silicon oxide or TEOS oxide. In otherembodiments, bottom structural layer 152 and top structural layer 160are other types of dielectric material or structural material, such asdescribed hereinabove in reference to the insulating layer of step 210or the substrate in FIG. 5. Bottom stress layer 154 and top stress layer158 include any of the materials as described hereinabove in referenceto the first stress layer of step 215 and the second stress layer ofstep 235 in FIG. 5. For example, in a specific embodiment, bottom stresslayer 154 and top stress layer 158 are formed of silicon nitride. Invarious embodiments, conductive layer 156 includes any of the materialsas described hereinabove in reference to the conductive material of step225 in FIG. 5. For example, in a specific embodiment, conductive layer156 is formed of polysilicon.

According to various embodiments, extension portion 150 a includesbalanced bottom stress layer 154 and top stress layer 158. The layerstress of bottom stress layer 154 is balanced by the layer stress of topstress layer 158. In such embodiments, the layer stress of bottom stresslayer 154 and top stress layer 158 is tensile or compressive. Further,in order to balance the layer stress, the bottom stress layer 154 andtop stress layer 158 is about equal.

In various embodiments, extension portion 150 b includes patternedopening 164 in bottom stress layer 154 and patterned opening 162 in topstress layer 158. For example, patterned opening 162 and patternedopening 164 may correspond to any of patterned opening 122, patternedopening 124, patterned opening 126, and patterned opening 128. Further,patterned opening 162 and patterned opening 164 may be formed asdescribed hereinabove in reference to step 220 in FIG. 5. As shown, dueto patterned opening 164, conductive layer 156 may include a bend orbump at patterned opening. Similarly, due to patterned opening 162, topstress layer 158 may include a bend or bump above patterned opening 164.

In various embodiments, in order to remove the bend or bump inconductive layer 156 and top stress layer 158, extension portion 150 cincludes fill material 168 in place of patterned opening 164. Fillmaterial 168 may have the same shape as patterned opening 164 and may beformed in the same way, but includes a further step of filling thepatterned opening with fill material 168. In such embodiments, fillmaterial 168 is removed during a release etch, such as describedhereinabove in reference to step 250 in FIG. 5. Fill material 168 mayinclude an oxide, nitride, or oxynitride in various embodiments. In aspecific embodiment fill material 168 is a TEOS oxide. In variousembodiments including fill material 168, a chemical mechanical polish(CMP) process may be applied to planarize the surface at intermediatefabrication steps. For example, a CMP process may be applied afterforming bottom stress layer 154 with the patterned opening anddepositing fill material 168. In various embodiments, dip 166 is adepression, bend, or hole in top structural layer 160 over the patternedopening in top stress layer 158.

According to various embodiments, extension portions 150 d, 150 e, and150 f correspond to extension portions 150 a, 150 b, and 150 c, but eachincludes a thicker conductive layer 156. In such embodiments, thedescription provided hereinabove in reference to extension portions 150a, 150 b, and 150 c applies to the similarly numbered elements ofextension portions 150 a, 150 b, and 150 c and will not be repeated inthe interest of brevity. In various embodiments, the thickness ofconductive layer 156 may range from 100 nm to 2 μm. In more specificembodiments, the thickness of conductive layer 156 ranges from 200 nm to800 nm. In a specific embodiment, such as depicted by extension portions150 a, 150 b, and 150 c, conductive layer 156 has a thickness of 150 nm.In another specific embodiment, such as depicted by extension portions150 d, 150 e, and 150 f, conductive layer 156 has a thickness of 660 nm.

In various embodiments, the thicknesses of top stress layer 158 andbottom stress layer 154 may range from 50 nm to 1 μm. In more specificembodiments, the thicknesses of top stress layer 158 and bottom stresslayer 154 range from 100 nm to 500 nm. In a specific embodiment, such asdepicted by extension portions 150 a, 150 b, 150 c, 150 d, 150 e, and150 f top stress layer 158 and bottom stress layer 154 have thicknessesof 140 nm.

FIG. 7 illustrates a cross sectional view of still another embodimenttransducer 170, which is similar to transducer 110 a, transducer 110 b,transducer 110 c, and transducer 110 d as described hereinabove inreference to FIGS. 2 a, 2 b, 2 c, 3 a, 3 b, and 4, with the addition ofconductive or isolation layer 172 and substrate 174. Although transducer170 is shown without the complete membrane 114, interdigitated combfingers 120 a, and interdigitated comb fingers 120 b, these elements areincluded in transducer 170, but are omitted from the drawing in order tosimplify the illustration. Specifically, each of the elements describedhereinabove in reference to transducer 110 a, transducer 110 b,transducer 110 c, and transducer 110 d in FIGS. 2 a, 2 b, 2 c, 3 a, 3 b,and 4 also applies to similarly numbered elements in FIG. 7.

Substrate 174 may be a semiconductor substrate, such as silicon. Invarious embodiments, any of the materials described hereinabove inreference to the substrate of method of fabrication 200 in FIG. 5 mayalso be used for substrate 174. In such embodiments, substrate 112 is astructural material, such as a TEOS oxide that is formed and patternedto support positive stator 118, negative stator 116, and membrane 114.Further, cavity 113 is formed in both substrate 174 and substrate 112 bya backside etch process, such as described hereinabove in reference tostep 245 in FIG. 5.

In various embodiments, the backside etch process may form a roughsidewall in substrate 174 and substrate 112 without precisely controlleddimensions, such as a diameter. In such embodiments, conductive layer172 may be patterned more precisely to clearly define the dimensions ofthe opening, such as the diameter, in order to more clearly control theelectrical characteristics of the fabricated transducer by shieldingpositive stator 118, negative stator 116, and membrane 114 from theeffects of the rough sidewalls in substrate 174 and substrate 112. Insome embodiments, conductive layer 172 is polysilicon. In otherembodiments, conductive layer 172 is a metal, such as copper, aluminum,gold, or platinum, for example. Conductive layer 172 may include any ofthe materials described hereinabove in reference to the conductivematerial of step 225 in FIG. 5. According to various embodiments,conductive layer 172 and substrate 174 may be included in any of theembodiment transducers described herein, such as transducer 110 a,transducer 110 b, transducer 110 c, and transducer 110 d, for example.

FIG. 8 illustrates a flow chart diagram of another embodiment method offabrication 300 for an embodiment transducer. Method of fabrication 300includes steps 305, 310, 315, 320, 325, 330, and 335. According tovarious embodiments, method of fabrication 300 is a method of formingany of transducer 110 a, transducer 110 b, transducer 110 c, andtransducer 110 d, for example. In various embodiments, step 305 includesforming a plurality of trenches in a substrate. Step 310 includesforming comb fingers in the plurality of trenches. Following, orsimultaneous with, step 310, step 315 includes forming a membraneconnected to a first subset of the comb fingers.

In various embodiments, step 320 includes forming a first extensionlayer connected to a second subset of the comb fingers, where the firstextension layer includes a first intrinsic stress. Step 325 includesforming a second extension layer connected to a third subset of the combfingers, where the second extension layer includes a second intrinsicstress. The first intrinsic stress and the second intrinsic stress maybe compressive stress or tensile stress. Further, the first intrinsicstress and the second intrinsic stress may each be affected bypatterning in the first extension layer and the second extension layer.For example, the first extension layer and the second extension layermay each include multiple layers and the multiple layers may bepatterned to with different patterns in order to produce a layerstresses that cause different at rest deflections of the first extensionlayer and the second extension layer.

According to various embodiments, step 330 includes forming a cavity inthe substrate beneath the comb fingers, the membrane, the firstextension layer, and the second extension layer. Step 335 includesreleasing the membrane, the first extension layer, and the secondextension layer in a release etch. In such embodiments, the firstintrinsic stress causes the first extension layer to deflect in a firstdirection during the release etch and the second intrinsic stress causesthe second extension layer to deflect in a second direction during therelease etch. The second direction is different from the firstdirection. For example, the first direction may be upward and the seconddirection may be downward.

In various embodiments, any of steps 305, 310, 315, 320, 325, 330, and335 may include details as described hereinabove in reference to methodof fabrication 200 in FIG. 5. Further, steps 305, 310, 315, 320, 325,330, and 335 may modified, substituted, and rearranged based ondifferent embodiments, as will be readily appreciated by those havingskill in the art. Additional steps may also be added to method offabrication 300.

According to an embodiment, a MEMS device includes a deflectablemembrane including a first plurality of electrostatic comb fingers, afirst anchor structure including a second plurality of electrostaticcomb fingers interdigitated with a first subset of the first pluralityof electrostatic comb fingers, and a second anchor structure including athird plurality of electrostatic comb fingers interdigitated with asecond subset of the first plurality of electrostatic comb fingers. Thesecond plurality of electrostatic comb fingers are offset from the firstplurality of electrostatic comb fingers in a first direction and thethird plurality of electrostatic comb fingers are offset from the firstplurality of electrostatic comb fingers in a second direction, where thefirst direction is different from the second direction. Otherembodiments of this aspect include corresponding systems, apparatus, andprocessors, each configured to perform embodiment methods.

In various embodiments, the deflectable membrane extends over a firstplane and the first direction and the second direction both include anout-of-plane component for the first plane. In some embodiments, thedeflectable membrane includes one of a polygon membrane, a roundmembrane, and an oval membrane. Specifically, the deflectable membranemay include a rectangular membrane. The deflectable membrane may includean octagonal membrane.

In various embodiments, the deflectable membrane is anchored to asupport structure along a first edge of the deflectable membrane. Insuch embodiments, the first plurality of electrostatic comb fingers maybe connected to a second edge of the deflectable membrane, where thefirst edge and the second edge are on opposite sides of the deflectablemembrane. In other embodiments, the deflectable membrane is anchored toa first support structure along a first edge of the deflectable membraneand is anchored to a second support structure along a second edge of thedeflectable membrane. In still other embodiments, the deflectablemembrane is anchored to a first support structure, a second supportstructure, a third support structure, and a fourth support structure atfour corners, respectively, of the deflectable membrane.

In various embodiments, the first anchor structure further includes afirst extension portion connected to the second plurality ofelectrostatic comb fingers, the first extension portion having a firstinternal stress configured to offset the second plurality ofelectrostatic comb fingers from the first plurality of electrostaticcomb fingers in the first direction, and the second anchor structurefurther includes a second extension portion connected to the thirdplurality of electrostatic comb fingers, the second extension portionhaving a second internal stress configured to offset the third pluralityof electrostatic comb fingers from the first plurality of electrostaticcomb fingers in the second direction. In such embodiments, the firstextension portion includes two material layers configured to generatethe first internal stress, and the second extension portion includes twomaterial layers configured to generate the second internal stress. Thefirst extension portion and the second extension portion may include twomaterial layers of a same two materials, where a first material of thesame two materials includes polysilicon and a second material of thesame two materials includes silicon nitride. In other embodiments, thefirst extension portion and the second extension portion may include twomaterial layers of a same two materials, where a first material of thesame two materials includes a metal and a second material of the sametwo materials includes an insulator.

In various embodiments, the MEMS device further includes a substrateincluding a cavity, where the cavity underlies the deflectable membrane.In such embodiments, the MEMS device further includes a support layerformed around the cavity and supporting the deflectable membrane, thefirst anchor structure, and the second anchor structure. The MEMS devicefurther includes a conductive layer formed in the support layer aroundthe cavity and extending into the cavity in such embodiments.

According to an embodiment, a MEMS device includes a membrane includinga diaphragm portion and a first comb finger portion, a first anchorstructure, and a second anchor structure. The first comb finger portionincludes a first plurality of electrostatic comb fingers. The firstanchor structure includes a first anchor portion fixed to a substrate, afirst extension portion extending away from the first anchor portion,and a second comb finger portion including a second plurality ofelectrostatic comb fingers interdigitated with a first subset of thefirst plurality of electrostatic comb fingers. The first extensionportion includes a first material with a first intrinsic stress thatcauses the first extension portion to deflect in a first direction. Thesecond anchor structure includes a second anchor portion fixed to thesubstrate, a second extension portion extending away from the secondanchor portion, and a third comb finger portion including a thirdplurality of electrostatic comb fingers interdigitated with a secondsubset of the first plurality of electrostatic comb fingers. The secondextension portion includes a second material with a second intrinsicstress that causes the second extension portion to deflect in a seconddirection, where the second direction is opposite the first direction.Other embodiments of this aspect include corresponding systems,apparatus, and processors, each configured to perform embodimentmethods.

In various embodiments, the diaphragm portion includes one of a polygondiaphragm, a round diaphragm, and an oval diaphragm. Specifically, thediaphragm portion includes an octagonal diaphragm. In another specificembodiment, the diaphragm portion includes a rectangular diaphragm. Insuch embodiments, the rectangular diaphragm is anchored to a thirdanchor structure along a first edge of the rectangular diaphragm, andthe first comb finger portion is connected to the rectangular diaphragmalong a second edge of the rectangular diaphragm, where the first edgeis opposite the second edge.

In various embodiments, the first material includes a first plurality ofmaterial layers that together have the first intrinsic stress, at leastone of the first plurality of material layers being patterned, and thesecond material includes a second plurality of material layers thattogether have the second intrinsic stress, at least one of the secondplurality of material layers being patterned. In such embodiments, thefirst plurality of material layers includes a top insulating layer, amiddle conductive layer, and a bottom insulating layer, where the topinsulating layer and the bottom insulating layer of the first pluralityof material layers are patterned according to different mask patterns.Further, in such embodiments, the second plurality of material layersincludes a top insulating layer, a middle conductive layer, and a bottominsulating layer, where the top insulating layer and the bottominsulating layer of the second plurality of material layers arepatterned according to different mask patterns.

According to an embodiment, a differential MEMS acoustic transducerincludes a first anchor, a deflectable membrane, and a firstdifferential electrostatic comb finger drive connected to thedeflectable membrane and to the first anchor. The first differentialelectrostatic comb finger drive includes a plurality of interdigitatedelectrostatic comb fingers including a first portion with a first offsetbetween the plurality of interdigitated electrostatic comb fingers, anda second portion with a second offset between the plurality ofinterdigitated electrostatic comb fingers. The first offset is in adifferent direction than the second offset. Other embodiments of thisaspect include corresponding systems, apparatus, and processors, eachconfigured to perform embodiment methods.

In various embodiments, the deflectable membrane includes one of apolygon membrane, a circular membrane, and an oval membrane.Specifically, the deflectable membrane may include a rectangularmembrane. In some embodiments, the rectangular membrane includes arectangular flap membrane anchored to a second anchor on a first edge ofthe rectangular flap membrane and connected to the first differentialelectrostatic comb finger drive on a second edge of the rectangular flapmembrane.

In various embodiments, the differential MEMS acoustic transducerfurther includes a second differential electrostatic comb finger driveconnected to the deflectable membrane and to a second anchor. In suchembodiments, the second differential electrostatic comb finger driveincludes a plurality of interdigitated electrostatic comb fingersincluding a first portion with a first offset between the plurality ofinterdigitated electrostatic comb fingers, and a second portion with asecond offset between the plurality of interdigitated electrostatic combfingers, where the first offset is in a different direction than thesecond offset. The rectangular membrane may be anchored to a thirdanchor on a first edge of the rectangular membrane, anchored to a fourthanchor on a second edge of the rectangular membrane, connected to thefirst differential electrostatic comb finger drive on a third edge ofthe rectangular membrane, and connected to the second differentialelectrostatic comb finger drive on a fourth edge of the rectangularmembrane. The second edge is opposite the first edge and the fourth edgeis opposite the third edge in such embodiments.

In various embodiments, the differential MEMS acoustic transducerfurther includes a second differential electrostatic comb finger driveconnected to the deflectable membrane and to a second anchor, the seconddifferential electrostatic comb finger drive including a plurality ofinterdigitated electrostatic comb fingers including a first portion witha first offset between the plurality of interdigitated electrostaticcomb fingers, and a second portion with a second offset between theplurality of interdigitated electrostatic comb fingers, where the firstoffset is in a different direction than the second offset. In suchembodiments, the differential MEMS acoustic transducer further includesa third differential electrostatic comb finger drive connected to thedeflectable membrane and to a third anchor, the third differentialelectrostatic comb finger drive including a plurality of interdigitatedelectrostatic comb fingers including a first portion with a first offsetbetween the plurality of interdigitated electrostatic comb fingers, anda second portion with a second offset between the plurality ofinterdigitated electrostatic comb fingers, where the first offset is ina different direction than the second offset. In such embodiments, thedifferential MEMS acoustic transducer further includes a fourthdifferential electrostatic comb finger drive connected to thedeflectable membrane and to a fourth anchor, the fourth differentialelectrostatic comb finger drive including a plurality of interdigitatedelectrostatic comb fingers including a first portion with a first offsetbetween the plurality of interdigitated electrostatic comb fingers, anda second portion with a second offset between the plurality ofinterdigitated electrostatic comb fingers, where the first offset is ina different direction than the second offset. In such embodiments, therectangular membrane is anchored to a fifth anchor at a first corner ofthe rectangular membrane, anchored to a sixth anchor at a second cornerof the rectangular membrane, anchored to a seventh anchor at a thirdcorner of the rectangular membrane, anchored to an eighth anchor at afourth corner of the rectangular membrane, connected to the firstdifferential electrostatic comb finger drive on a first edge of therectangular membrane, connected to the second differential electrostaticcomb finger drive on a second edge of the rectangular membrane,connected to the third differential electrostatic comb finger drive on athird edge of the rectangular membrane, and connected to the fourthdifferential electrostatic comb finger drive on a fourth edge of therectangular membrane.

According to an embodiment, a method of fabricating a MEMS deviceincludes forming a plurality of trenches in a substrate; forming combfingers in the plurality of trenches; forming a membrane connected to afirst subset of the comb fingers; forming a first extension layerconnected to a second subset of the comb fingers, where the firstextension layer including a first intrinsic stress; forming a secondextension layer connected to a third subset of the comb fingers, wherethe second extension layer including a second intrinsic stress; forminga cavity in the substrate beneath the comb fingers, the membrane, thefirst extension layer, and the second extension layer; and releasing themembrane, the first extension layer, and the second extension layer in arelease etch. The first intrinsic stress causes the first extensionlayer to deflect in a first direction during the release etch and thesecond intrinsic stress causes the second extension layer to deflect ina second direction during the release etch, where the second directionis different from the first direction. Other embodiments of this aspectinclude corresponding systems, apparatus, and processors, eachconfigured to perform embodiment methods.

In various embodiments, forming comb fingers in the plurality oftrenches includes depositing an insulating layer on sidewalls andbottoms of the plurality of trenches and depositing a conductivematerial in the trenches. In such embodiments, forming the membraneconnected to the first subset of the comb fingers may include depositingthe conductive material on a flat portion of the substrate during a samedeposition step as depositing the conductive material in the trenchesand patterning the conductive material on the flat portion of thesubstrate to form the membrane. In some embodiments, forming the firstextension layer and forming the second extension layer includedepositing a first insulating layer, patterning the first insulatinglayer with a first pattern, depositing a first conducting layer,depositing a second insulating layer, and patterning the secondinsulating layer with a second pattern, where the second pattern isdifferent from the first pattern.

According to some embodiments described herein, advantages of embodimenttransducers may include low acoustic noise, low risk of pull-in,differential transduced output signals, high sensitivity levels, andlarge dynamic range.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A microelectromechanical systems (MEMS) devicecomprising: a deflectable membrane comprising a first overlappingportion, the first overlapping portion comprising a first surface; afirst anchor structure comprising: a first anchor portion fixed to asubstrate, a first extension portion extending away from the firstanchor portion, the first extension portion comprising a first material,wherein the first extension portion bends in a first direction, and asecond overlapping portion comprising a second surface overlapping thefirst surface; and a second anchor structure comprising: a second anchorportion fixed to the substrate, a second extension portion extendingaway from the second anchor portion, the second extension portioncomprising a second material, wherein the second extension portion bendsin a second direction, the second direction opposite the firstdirection, and a third overlapping portion comprising a third surface,wherein the first overlapping portion further comprises a fourthsurface, and wherein the third surface overlaps the fourth surface. 2.The MEMS device of claim 1, wherein: the deflectable membrane extends ina first plane; the first direction comprises a first out-of-planecomponent of the first plane; and the second direction comprises asecond out-of-plane component of the first plane.
 3. The MEMS device ofclaim 1, wherein the deflectable membrane is anchored to a first supportstructure, a second support structure, a third support structure, and afourth support structure at four corners, respectively, of thedeflectable membrane.
 4. The MEMS device of claim 1, wherein the firstextension portion and the second extension portion each comprise twomaterial layers.
 5. The MEMS device of claim 1, wherein the deflectablemembrane comprises one of a polygon membrane, a round membrane, and anoval membrane.
 6. The MEMS device of claim 5, wherein the deflectablemembrane comprises an octagonal membrane.
 7. The MEMS device of claim 5,wherein the deflectable membrane comprises a rectangular membrane. 8.The MEMS device of claim 7, wherein the rectangular membrane is anchoredto a third anchor structure along a first edge of the rectangularmembrane, and the first overlapping portion is connected to therectangular membrane along a second edge of the rectangular membrane,the first edge being opposite the second edge.
 9. The MEMS device ofclaim 1, wherein the first material comprises a first plurality ofmaterial layers that together bend in the first direction, at least oneof the first plurality of material layers being patterned, and thesecond material comprises a second plurality of material layers thattogether bend in the second direction, at least one of the secondplurality of material layers being patterned.
 10. The MEMS device ofclaim 9, wherein the first plurality of material layers comprises a topinsulating layer, a middle conductive layer, and a bottom insulatinglayer, the top insulating layer and the bottom insulating layer of thefirst plurality of material layers being patterned according todifferent mask patterns, and the second plurality of material layerscomprises a top insulating layer, a middle conductive layer, and abottom insulating layer, the top insulating layer and the bottominsulating layer of the second plurality of material layers beingpatterned according to different mask patterns.
 11. A method offabricating a microelectromechanical systems (MEMS) device, the methodcomprising: forming a first material layer at a first surface of asubstrate; forming a conductive layer over the first material layer;patterning the conductive layer to form a membrane connected to a firstoverlapping portion, a first extension region, the first extensionregion connected to a second overlapping portion that overlaps the firstoverlapping portion in a plane of the first surface, and a secondextension region, the second extension region connected to a thirdoverlapping portion that overlaps the first overlapping portion in theplane of the first surface; forming a second material layer over theconductive layer; and releasing the membrane, the first extensionregion, and the second extension region in a release etch, wherein thereleasing causes the first extension region to deflect in a firstdirection during the release etch, and the releasing causes the secondextension region to deflect in a second direction during the releaseetch, the second direction different from the first direction.
 12. Themethod of claim ii, wherein: forming the first material layer comprisesdepositing the first material layer over the first surface of thesubstrate, forming the second material layer comprises depositing thesecond material layer over the conductive layer, and the first materiallayer and the second material layer each comprise a material selectedfrom a group consisting of silicon nitride (SiN), silicon oxynitride(SiON), and tetraethyl orthosilicate (TEOS).
 13. The method of claim 11,further comprising: patterning the first material layer with a firstpattern to form first patterned openings; and patterning the secondmaterial layer with a second pattern to form second patterned openings,the first patterned openings and the second patterned openings causingthe first extension region to deflect in the first direction and thesecond extension region to deflect in the second direction afterreleasing the membrane, the first extension region, and the secondextension region.
 14. The method of claim 13, wherein the first patternis different from the second pattern.
 15. The method of claim 11,further comprising: before forming the first material layer, forming aninsulating layer at a substrate; and after forming the second materiallayer, etching the substrate from a second surface opposite the firstsurface to form a cavity, wherein releasing the membrane, the firstextension region, and the second extension region comprises etching theinsulating layer.
 16. A method of fabricating a microelectromechanicalsystems (MEMS) device, the method comprising: forming a plurality oftrenches in a substrate; forming a membrane overlapping a first subsetof the plurality of trenches; forming a first extension regionoverlapping a second subset of the plurality of trenches, the firstextension region; forming a second extension region overlapping a thirdsubset of the plurality of trenches, the second extension region; andreleasing the membrane, the first extension region, and the secondextension region in a release etch, wherein the releasing causes thefirst extension region to deflect in a first direction during therelease etch, and the releasing causes the second extension region todeflect in a second direction during the release etch, the seconddirection different from the first direction.
 17. The method of claim16, further comprising: forming a cavity in the substrate beneath themembrane, the first extension region, and the second extension region.18. The method of claim 16, further comprising: depositing an insulatinglayer on sidewalls and bottoms of the plurality of trenches; anddepositing a conductive material in the trenches.
 19. The method ofclaim 18, wherein forming the membrane overlapping the first subset ofthe plurality of trenches comprises: depositing the conductive materialon a flat portion of the substrate during a same deposition step asdepositing the conductive material in the trenches; and patterning theconductive material on the flat portion of the substrate to form themembrane.
 20. The method of claim 16, wherein forming the firstextension region and forming the second extension region comprise:depositing a first insulating layer, patterning the first insulatinglayer with a first pattern, depositing a first conducting layer,depositing a second insulating layer, and patterning the secondinsulating layer with a second pattern, the second pattern differentfrom the first pattern.